In U.S. Pat. No. 2,326,145 issued in 1943, it was proposed to fasten segmental blade groups to turbine rotors by two transverse pins extending through openings provided in or near to the end faces of the root portion of the blade group. During use, it was found that this arrangement of root fastening suffered from metal creep, i.e., the metal in the blade root tended to flow around the two pins and the concentrated pin forces resulted in bulging of the rotor surface In order to alleviate the creep problem, the fastening arrangement was modified by extending the depth of the root portion sufficiently to allow a third pin to be inserted. The third pin was positioned below the diameter of the original two pins so that a triangular arrangement of pins was created. This form of blade groups attachment has been in use for at least thirty years.
While the three pin fastening arrangement has been generally satisfactory, it is desirable to improve the vibratory strength of such segmental blade groups and to simplify the assembly of the groups to a turbine rotor. For example, the prior art blade groups generally incorporated three blades in a group with three fastening pins. This generally necessitated smaller pins and correspondingly smaller diameter holes in the blade root and rotor flanges. With holes in the order of one-half inch in diameter, it is difficult to maintain an exact hole when the hole is drilled through three flanges on the blade root and four flanges on the rotor, i.e., it is not uncommon for a drill bit to wander slightly when drilling extended holes. This action may create pin fit problems as well as detrimentally affecting loading on the pin. Still further, with off-set pins, i.e., at different radial distances from the rotor, thermal expansion at the outer pins is greater than the inner pin thus placing more load on the inner pin. Since the centrifugal loading on a blade group may be as much as 100,000 pounds, uneven loading on the pins may lead to shorter service life.